Standard optical surfaces include anti-reflective coatings as well as other coatings for beam splitters, mirrors, charge-coupled devices (CCDs), detectors, and time delay integration (TDI) CCDs. Unfortunately, these coatings may be adversely affected by humidity, oxidation, contamination, radiation damage, and other environmental conditions. Specifically, various environmental conditions can deteriorate the coating performance or at least induce coating damage. Moreover, high fluence applications (i.e. where the total number of photons intersecting a unit area in a specific time interval is high) typically expose optical surfaces to radiation for long periods of time, which can exacerbate deterioration and damage of those coatings.
Various bulk materials have been suggested for protecting optical surfaces. One exemplary material suggesting for protecting optical surfaces used in UV (ultra-violet) 193 nm and 157 nm applications is fluorine-doped silica glass. FIG. 1 illustrates an exemplary process for manufacturing a protective layer from fused silica. This process uses a precursor material 101 of high purity fused silica. A natural silica precursor is typically melted in a furnace, either electrically or with a H2/O2 flame, and then grown to form ingots or large bulk material. A synthetic fused silica is made from a silicon-rich chemical precursor using, for example, a continuous flame hydrolysis process. This process includes the chemical gasification of silicon, the oxidation of this gas to silicon oxide, and the thermal fusion of the resulting dust. Yet another fused silica can be formed by adding silicon tetrachloride to a hydrogen-oxygen flame.
In step 102, the precursor material can be melted (or processed) and then doped with fluorine to form large bulk structures (e.g. ingots). In step 103, the bulk structures can be cut into rough shapes depending on their final application. In step 104, the shapes are ground to a rough surface finish. In step 105, the material is polished to a final RMS (a standard industry surface roughness measured in microinches) roughness to meet optical specification.
The resulting optical elements (called lens, filters, pellicles, covers, layers, etc. in the industry) can provide protection from environmental conditions and offer long term high radiation resistance. However, their manufacture for use with specific optical components having custom sizes and shapes is time- and labor-intensive, thereby making the protected optical components more expensive.
Therefore, a need arises for a way to protect optical components from environmental conditions and to ensure long term high radiation resistance at minimum expense.